Refrigerated air driers are well known in the art of air conditioning. In a refrigerated air dryer system, warm, moist air such as from the interior of a factory, and which typically is compressed, is cooled and dried and then conveyed to a location where it is used. In such a compressed air system, it is important to reduce the water content of the compressed air before delivering the compressed air to the points of use to avoid condensation of moisture upon adiabatic decompression. This is known in the prior art to be accomplished by using air- or water-cooled aftercoolers, moisture separators, and air dryers. Air dryers are available in many different types, and the present invention is illustrated with a non-cycling direct expansion refrigerated air dryer wherein the compressor operates continuously. This type of air dryer effectively reduces water content in compressed air by physically chilling the compressed air directly with a refrigeration circuit and thus reducing the capacity of the compressed air to hold water vapor. Water vapor in the chilled compressed air condenses as liquid droplets as the temperature of the compressed air is lowered to a desired dew point, typically about 40° F. The combination of chilled air and water droplets flows through a moisture separator that mechanically removes the droplets from the air stream.
It is further desirable to reheat, or “temper,” the dried air to lower the relative humidity and thereby prevent formation of condensation at the use point, and also to prevent atmospheric condensation on compressed air piping within the factory, as might occur if the chilled dried air were piped directly without insulation.
The main components in this type of refrigerated air dryer are a refrigeration system, a moisture separator, and two air heat exchangers.
The first of these heat exchangers is a precooler/reheater. It precools warm saturated compressed air from an air compressor aftercooler by transferring heat to chilled air that is being returned from the moisture separator. One benefit of this heat exchanger is that it reduces some of the cooling load that the refrigeration system would otherwise have to handle in subsequent dehumidification of the air. The refrigeration system becomes smaller, requiring less power for thriftier operation. The precooler/reheater heat exchanger is also known in the art as the “economizer” because of this benefit. Another benefit offered by this first heat exchanger is that it reheats the chilled air coming from the moisture separator, as described below. As noted above, reheating the chilled air reduces the chances that low ambient conditions can cause condensation in the air line downstream of the dryer and also reduces the likelihood of pipeline condensation or “sweating” that can occur on chilled surfaces in humid use conditions downstream of the PCR system.
The second heat exchanger is a refrigerant-to-air chiller that takes the precooled air from the first heat exchanger and chills it to the desired dewpoint temperature by transferring heat from the air into a cold refrigerant on the other side of the heat exchanger, thereby causing condensation of water from the air. After being thus chilled, the air enters a moisture separator to remove any remaining condensed water, and then the air is returned to the cold side of the first heat exchanger for reheating and exit from the PCR.
U.S. Pat. No. 5,845,505 and No. 6,085,529 disclose PCRs that function substantially as just described, and the relevant disclosures of these patents are herein incorporated by reference.
A prior art PCR in accordance with these disclosures comprises a precooler/reheater heat exchanger, also known as a precooler and reheater core, and a chiller heat exchanger, also known as a chiller core, in adjacent relation. Warm compressed air is passed through the precooler/reheater heat exchanger, and then directly through the chiller heat exchanger serially in a first direction, and then through a moisture separation means wherein water condensed in the chiller heat exchanger is collected and drained. The dried, cooled air is collected and directed via a return manifold to pass through the reheater side of the precooler/reheater heat exchanger in a second direction substantially perpendicular to the first direction.
Such a prior art PCR, although functionally effective, is relatively costly to manufacture and is not readily scalable using modular components, for the following reasons:
The prior art PCR design requires that the precooler/reheater core sections and the chiller core sections be stacked and brazed in two separate operations. After brazing is completed, manifolds are welded to the individually brazed cores, and then the individual cores and moisture separator sections are welded together to complete the assembly. What is needed in the art is an arrangement wherein all three sections may be stacked and brazed efficiently in one sub-assembly.
The prior art PCR requires a large and complex return manifold to direct the compressed air flow from the moisture separation section to the reheater section. This manifold is typically a large and complex aluminum casting, the size of which cannot be readily altered to accommodate larger or smaller capacity heat exchangers as may be desired for various end-use applications. What is further needed in the art is a simple, straight manifold, preferably formed of off-the-shelf stock, which may be easily shortened or lengthened to accommodate heat exchangers of greater or lesser capacity.
The prior art PCR employs a five-piece refrigerant inlet manifold assembly to direct refrigerant flow into the chiller. An associated refrigerant system must include an expansion device such as a thermo-expansion valve or capillary tubes. What is further needed in the art is an inlet manifold assembly wherein chilled liquid refrigerant is both flashed and distributed into the chiller heat exchanger without resort to other expansion devices.
The prior art PCR employs cross-flow directions in both the precooler/reheater heat exchanger and the chiller heat exchanger. It is known that counter-flow heat exchangers can be more efficient and provide more uniform temperature profiles in the fluids within the heat exchangers. This is especially important in the refrigerant side of the chiller, as it is more likely to provide uniformly superheated refrigerant vapor exiting the chiller, thereby reducing the likelihood that any liquid refrigerant can leave the chiller and enter the refrigerant compressor downstream, and thereby improving the overall control of the air dryer operation. What is further needed in the art is a PCR wherein all heat exchange is performed in a counter-flow fashion.
The prior art PCR has the precooler/reheater heat exchanger in mechanical contact with the chiller heat exchanger along their mutual length, allowing heat transfer therebetween which compromises the thermal efficiency of both and increases the thermal load on the refrigerant system. What is needed in the art is a means for insulating the precooler/reheater heat exchanger from the chiller to allow an air dryer manufacturer to reduce the capacity and cost of the appropriate refrigeration system for drying capacity of a given air dryer system.
The prior art PCR uses means for moisture separation, including a mesh pad inserted next to the chiller to capture and coalesce much of the condensed water leaving the chiller. Also, the return manifold is used to reduce the vertical upward velocity of the air flow and allow gravity to separate out any remaining droplets leaving the mesh pad. The manifold must be large enough to accommodate the mesh pad and also to reduce the upward air velocity sufficiently to prevent carryover of water into the reheater. Because the air within the PCR is compressed typically to 100 psig or more, the manifold, being of irregular shape not optimized for burst resistance, must be formed with very thick, heavy walls reinforced by internal bars. What is needed in the art is an air flow pathway and apparatus wherein condensate is readily removed without requiring a large, heavy, and expensive manifold.
It is a primary object of the invention to provide a precooler/chiller/reheater system having counterflow heat exchangers that is readily adaptable to various sizes and capacities of air flow and heat exchange.